Field of the Invention
This invention is concerned with impact printing devices such as typewriters and, in particular, with such mechanisms in which the printing is achieved by a rotary device such as a daisy-type wheel or a cup-shaped printing element. Such elements typically include a plurality of spokes or petals each bearing a character or characters thereon.
A printing element of this type is commonly located on a carrier for movement along a print line direction during the printing operation. As the carrier is moved from print position to print position along the print line direction, the printing element is rotated about its axis for character selection purposes. When the printing element is in its proper position (i.e. when the character on the printing element to be printed is opposite the desired print position), the carrier is momentarily stopped and a print hammer on the carrier impresses the character on the printing element against an inking ribbon and a print receiving medium, such as paper.
After a printing operation occurs, the print hammer returns to its rest position and the printing element is rotated so that the next character on the printing element to be printed is opposite the desired print position. The next, and subsequent, printing steps occur in the same manner as described above, until a complete line has been printed. After printing a complete line, appropriate means are provided to move the carrier to the next starting line position for subsequent printing. A printing device which operates in this manner is the typewriter sold by SCM Corporation under the model "XE 5000."
In printing devices of this type, it is necessary to change the printing element from time to time when, for example, a new type font is desired or if the printing element becomes damaged. When that occurs, it is essential that means be provided to assure that the printing element is engaged and maintained in its home position after the printing element is inserted, and also that the printing element be in its correct escapement position after the printing element is inserted. Furthermore, it is highly desirable that the maintenance of the printing element in its home position be achieved without any accompanying noise. The purpose of the present invention is to provide such means.